> I suspect that SYNC insn alone is still not enough, insn't it? In> such systems with that 'deep' write buffer and data incoherency is> visibly observed, there sill may be data write transactions floating> in the internal bus system.

A SYNC in theory should ensure global visibilty of preceding writes andcompletion of earlier reads. That usually works between CPUs but notall I/O systems fully participate in that "consistency domain" so moreor less arbitary shaking of the I/O system may still be required to toachieve consistency.